Feeling a bit down today? Don't blame the date — blame the date's tendency of bringing up a tired, old and long-debunked theory about "the most depressing day of the year." 

Hundreds of international media outlets have referred to the third Monday of January as "Blue Monday" since 2005, when a British travel agency issued a press release declaring it so.

That release, issued by the now defunct Sky Travel holiday company, cited a pseudo-mathematical formula commissioned from a U.K. psychologist as proof that such factors as weather, debt levels and failed New Year's resolutions made this the most depressing time of the year.

British scientists started coming out against the "research" almost immediately, prompting Cardiff University to eventually issue a statement distancing itself from the "Cardiff University psychologist" who'd developed the formula.

"Cardiff University has asked us to point out that Dr. Cliff Arnall, mentioned in the article below, was a former part-time tutor at the university but left in February," reads a correction at the top of a 2006 piece from the Guardian's Bad Science column.

Arnall went on to promote a similar formula for the "happiest day of the year" as part of a paid gig for an ice cream company's marketing campaign, and was billing himself as a "freelance happiness guru" in 2010 when he admitted to the Telegraph that his Blue Monday theory was "not particularly helpful."

"I was originally asked to come up with what I thought was the best day to book a summer holiday," he said at the time. "But when I started thinking about the motives for booking a holiday … there were these factors that pointed to the third Monday in January as being particularly depressing," he said.

"But it is not particularly helpful to put that out there and say 'there you are'," he noted. "It is almost a self-fulfilling prophecy that it is the most depressing day." 

Indeed, it can be hard not to feel a bit glum when you've got every other news outlet, Facebook post and co-worker reminding you that January sucks.

And in fact, there is some truth to the claim that a lack of sunlight is linked to sadness.

But stories about Blue Monday lacking credibility have become nearly as common as junkweb listicles about "how to beat Blue Monday" in recent years, and yet we continue to see more content focused on this "most depressing day" pop up every January.

It's puzzling to many in science and journalism that the myth persists despite thorough evidence debunking it — not to mention additional, more recent layers of criticism over how Blue Monday's treatment makes light of mental illness, which can occur any time.

Why is Blue Monday still a thing?

One might presume that money has something to do with the theory's continuing popularity online, based on the number of promotions, campaigns and sponsored content pieces created for Blue Monday 2016.

Some companies stick with tweeting hip GIFs and pictures of baby animals to cheer up their followers on Blue Monday without pushing any particular deals.

This, while annoying to some, is relatively chill in terms of #BrandsOnTwitter.

Others promote special offers geared towards helping consumers "Beat the Blue Monday blahs," as Ottawa's Metro newspaper describes the Blue Monday deal this year at Cineplex movie theatres.

Some businesses have gone even further in promotions this year.

Toronto-based personal loans firm Borrowell commissioned its own Blue Monday survey "to understand how Canadians feel" about their credit card debt, and the Canadian marketing director for coupon purveyor Ebates wrote this promotion-loaded column about "how to perk up" Blue Monday for the Huffington Post.

A bus company in Scotland is promoting its own "super" bus driver for Blue Monday, as well as offering customers a buy-one-get-one deal.

"With Monday (Jan 18th) widely recognized as the most depressing day of the year (Blue Monday), a bus driver in Aberdeen is helping to bring back the smiles to customer faces by donning fancy dress to help them get from A to B," reads a release on First Aberdeen's website.

"The campaign aims to reward regular passengers during January and also inspire people who currently don't travel by bus to consider making it one of their resolutions in 2016."

Buying a tropical vacation from one of the many travel agencies promoting Blue Monday deals might not be the best way to alleviate any debt woes, but there is some truth to the claim that a lack of sunlight is linked to sadness.

Seasonal affective disorder (SAD), which is real, is also known as the winter blues and affects some 10 per cent of all Canadians in mild forms, according to the Canadian Mental Health Association.  

Major depressive disorder (often referred to simply as depression), on the other hand, is different from the blues, and doctors don't treat it with cheap movie tickets, coupons or transportation deals.

"Depression, also known as clinical or major depression, is a mood disorder that will affect one in eight Canadians at some point in their lives," reads CMHA's B.C. website. "It changes the way people feel, leaving them with mental and physical symptoms for long periods of time."

"One of the most important things to remember about depression is that people who have it can't just 'snap out of it' or make it go away," the health resource continues.

"It's a real illness, and the leading cause of suicide."